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/F0 10/Courier-Bold@0 SF(Internet Engineering Task Force)72 85 Q
(Sally Floyd)180 E 270(INTERNET-DRAFT Mark)72 98 R(Handley)6 E 180
(draft-ietf-dccp-problem-01.ps Eddie)72 111 R(Kohler)6 E(ICIR)480 124 Q
(23 June 2005)432 137 Q(Expires: December 2005)372 150 Q/F1 14
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(yright \(C\) The Internet Society \(2005\). All Rights Reserv)-.11 E
(ed.)-.165 E F2(Abstract)267.534 498.2 Q F3(This document gi)97 520.8 Q
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(1. Introduction)97 110 Q F0 11(......................)3.56 G F2(3)11.5
E(2. Problem Space)97 122 Q F0 11(.....................)6.48 G F2(3)11.5
E(2.1. Congestion Control for Unreliable T)112 134 Q(ransfer)-.35 E F0
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(3. Solution Space for Congestion Control of Unreliable Flo)97 182 Q(ws)
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(........)11.29 G F2(9)11.5 E(3.3.1. Modifying TCP?)127 290 Q F0 11
(.................)8.69 G F2(9)11.5 E(3.3.2. Unreliable V)127 302 Q
(ariants of SCTP?)-1.11 E F0 11(.............)12.6 G F2(10)6.5 E
(3.3.3. Modifying R)127 314 Q(TP?)-.6 E F0 11(.................)9.29 G
F2(11)6.5 E(3.3.4. Designing a Ne)127 326 Q 2.5(wT)-.25 G
(ransport Protocol)-2.85 E F0 11(...........)12.65 G F2(11)6.5 E
(4. Selling Congestion Control to Reluctant Applications)97 338 Q F0 11
(..........)3.82 G F2(11)6.5 E(5. Additional Design Considerations)97
350 Q F0 11(...............)13.14 G F2(12)6.5 E(6. T)97 362 Q
(ransport Requirements of Request/Response Applications)-.35 E F0 11
(........)8.65 G F2(12)6.5 E(7. Summary of Recommendations)97 374 Q F0
11(................)8.84 G F2(13)6.5 E(8. Ackno)97 386 Q(wledgements)
-.25 E F0 11(....................)1.32 G F2(14)6.5 E(9. Informati)97 398
Q .3 -.15(ve R)-.25 H(eferences).15 E F0 11(...................).52 G F2
(14)6.5 E(10. Authors' Addresses)97 410 Q F0 11(...................)10.1
G F2(15)6.5 E(11. Full Cop)97 422 Q(yright Statement)-.1 E F0 11
(..................)1.42 G F2(15)6.5 E(12. Intellectual Property)97 434
Q F0 11(...................)7.32 G F2(15)6.5 E F0(NO)72 450.6 Q(TE T)
-.44 E 2.75(OR)-.198 G(FC EDIT)-2.75 E(OR: PLEASE DELETE THIS NO)-.198 E
(TE UPON PUBLICA)-.44 E(TION.)-1.221 E
(Changes from draft-ietf-dccp-problem-00.txt:)72 467.2 Q 2.75(*U)72
483.8 S(pdated references, minor editing changes.)-2.75 E
(Changes from draft-\215o)72 500.4 Q(yd-dcp-problem-01.txt:)-.11 E 2.75
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(Changes in response to feedback from Spencer Da)72 550.2 Q(wkins:)-.165
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(\214culties with ECN" about the possibility that in times)-.275 E
(of congestion, routers w)72 642.6 Q(ould \214rst "turn of)-.11 E
(f" the use of ECN to UDP \215o)-.275 E(ws.)-.275 E(Flo)72 769 Q
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-2.75 E F1(2.)72 613.6 Q F2(Pr)5.5 E(oblem Space)-.252 E F0 1.76 -.88
(We p)100.8 630.2 T(ercei).88 E .33 -.165(ve a n)-.275 H
(umber of problems related to the use of unreliable data \215o).165 E
(ws in the)-.275 E 2.75(Internet. The)100.8 643.2 R(major issues are:)
2.75 E 7.15<8354>100.8 659.8 S
(he potential for non-congestion-controlled datagram \215o)-7.15 E
(ws to cause congestion collapse)-.275 E(of the netw)111.8 672.8 Q(ork.)
-.11 E 7.15<8354>100.8 689.4 S(he dif)-7.15 E
(\214culty of correctly implementing ef)-.275 E(fecti)-.275 E .33 -.165
(ve c)-.275 H(ongestion control mechanisms for).165 E
(unreliable datagram \215o)111.8 702.4 Q(ws.)-.275 E(Flo)72 769 Q
(yd/Handle)-.11 E(y/K)-.165 E 244.01(ohler Section)-.385 F 2.75(2. [P)
2.75 F(age 3])-.165 E 0 Cg EP
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/F0 11/Times-Roman@0 SF(INTERNET)72 49 Q 66.092(-DRAFT Expires:)-1.012 F
(December 2005)2.75 E(June 2005)116.406 E 7.15<8354>100.8 85 S(he lack \
of a standard solution for reliably transmitting congestion feedback fo\
r an)-7.15 E(unreliable data \215o)111.8 98 Q -.715(w.)-.275 G 7.15
<8354>100.8 114.6 S(he lack of a standard solution for ne)-7.15 E
(gotiating ECN usage for unreliable \215o)-.165 E(ws.)-.275 E 7.15<8354>
100.8 131.2 S
(he lack of a choice of TCP-friendly congestion control mechanisms.)
-7.15 E 1.76 -.88(We a)100.8 147.8 T
(ssume that most application writers w).88 E
(ould use congestion control for long-li)-.11 E -.165(ve)-.275 G(d).165
E(unreliable \215o)100.8 160.8 Q(ws if it w)-.275 E(as a)-.11 E -.275
(va)-.22 G(ilable in a standard, easy-to-use form.).275 E
(More minor issues include:)100.8 177.4 Q 7.15<8354>100.8 194 S(he dif)
-7.15 E(\214culty of deplo)-.275 E
(ying applications using UDP-based \215o)-.11 E(ws in the presence of)
-.275 E(\214re)111.8 207 Q -.11(wa)-.275 G(lls.).11 E 7.15<8354>100.8
223.6 S(he desire to ha)-7.15 E .33 -.165(ve a s)-.22 H(ingle w).165 E
(ay to ne)-.11 E(gotiate congestion control parameters for unreliable)
-.165 E<8d6f>111.8 236.6 Q
(ws, independently of the signalling protocol used to set up the \215o)
-.275 E -.715(w.)-.275 G 7.15<8354>100.8 253.2 S(he desire for lo)-7.15
E 2.75(wp)-.275 G(er)-2.75 E(-pack)-.22 E(et byte o)-.11 E -.165(ve)
-.165 G(rhead.).165 E(The subsections belo)100.8 269.8 Q 2.75(wd)-.275 G
(iscuss these problems of pro)-2.75 E(viding congestion control, tra)
-.165 E -.165(ve)-.22 G(rsing).165 E(\214re)100.8 282.8 Q -.11(wa)-.275
G(lls, and ne).11 E(gotiating parameters in more detail.)-.165 E 2.75
(As)5.5 G(eparate subsection also discusses)-2.75 E
(the problem of minimizing the o)100.8 295.8 Q -.165(ve)-.165 G
(rhead of pack).165 E(et headers.)-.11 E/F1 11/Times-Bold@0 SF(2.1.)72
325.4 Q/F2 13/Times-Bold@0 SF(Congestion Contr)5.5 E(ol f)-.234 E
(or Unr)-.325 E(eliable T)-.234 E(ransfer)-.962 E F0(This project aims \
to bring easy-to-use congestion control mechanisms to applications that)
100.8 342 Q(generate lar)100.8 355 Q(ge or long-li)-.198 E -.165(ve)
-.275 G 2.75<648d>.165 G -.275(ow)-2.75 G 2.75(so).275 G 2.75(fu)-2.75 G
(nreliable datagrams, such as RealAudio, Internet)-2.75 E(telephon)100.8
368 Q 1.43 -.715(y, a)-.165 H(nd multiplayer g).715 E(ames. Our moti)
-.055 E -.275(va)-.275 G(tion is to a).275 E -.22(vo)-.22 G
(id congestion collapse. \(The).22 E(short \215o)100.8 381 Q
(ws generated by request-response applications, such as DNS, SNMP)-.275
E 2.75(,a)-1.221 G(nd SIP)-2.75 E(,)-1.221 E(don')100.8 394 Q 2.75(tc)
-.198 G(ause congestion in practice, and an)-2.75 E 2.75(yc)-.165 G
(ongestion control mechanism w)-2.75 E(ould tak)-.11 E(e)-.11 E(ef)100.8
407 Q(fect between \215o)-.275 E
(ws, not within a single end-to-end transfer of information.\) Ho)-.275
E(we)-.275 E -.165(ve)-.275 G -.44(r,).165 G(before designing a congest\
ion control mechanism for these applications, we must)100.8 420 Q
(understand wh)100.8 433 Q 2.75(yt)-.055 G(he)-2.75 E 2.75(yu)-.165 G
(se unreliable datagrams in the \214rst place, lest we destro)-2.75 E
2.75(yt)-.11 G(he v)-2.75 E(ery)-.165 E(properties the)100.8 446 Q 2.75
(yr)-.165 G(equire.)-2.75 E(There are se)100.8 462.6 Q -.165(ve)-.275 G
(ral reasons wh).165 E 2.75(yp)-.055 G
(rotocols currently use UDP instead of TCP)-2.75 E 2.75(,a)-1.221 G
(mongst them:)-2.75 E 7.15<8353>100.8 479.2 S(tartup Delay: the)-7.15 E
2.75(yw)-.165 G(ish to a)-2.75 E -.22(vo)-.22 G
(id the delay of a three-w).22 E(ay handshak)-.11 E 2.75(eb)-.11 G
(efore initiating)-2.75 E(data transfer)111.8 492.2 Q(.)-.605 E 7.15
<8353>100.8 508.8 S(tatelessness: the)-7.15 E 2.75(yw)-.165 G(ish to a)
-2.75 E -.22(vo)-.22 G
(id holding connection state, and the potential state-).22 E
(holding attacks that come with this.)111.8 521.8 Q 7.15<8354>100.8
538.4 S(rading of Reliability ag)-7.535 E(ainst T)-.055 E
(iming: the data being sent is timely in the sense that if it)-.385 E
(is not deli)111.8 551.4 Q -.165(ve)-.275 G
(red by some deadline \(typically a small number of R).165 E
(TTs\) then the data will)-.66 E(not be useful at the recei)111.8 564.4
Q -.165(ve)-.275 G -.605(r.).165 G
(Of these issues, applications that generate lar)100.8 581 Q
(ge or long-li)-.198 E -.165(ve)-.275 G 2.75<648d>.165 G -.275(ow)-2.75
G 2.75(so).275 G 2.75(fd)-2.75 G(atagrams, such as)-2.75 E
(media transfer and g)100.8 594 Q
(ames, mostly care about controlling the tradeof)-.055 E 2.75(fb)-.275 G
(etween timing and)-2.75 E(reliability)100.8 607 Q 2.75(.S)-.715 G
(uch applications use UDP because when the)-2.75 E 2.75(ys)-.165 G
(end a datagram, the)-2.75 E 2.75(yw)-.165 G(ish to)-2.75 E(send the mo\
st appropriate data in that datagram. If the datagram is lost, the)100.8
620 Q 2.75(ym)-.165 G(ay or may)-2.75 E(not resend the same data, depen\
ding on whether the data will still be useful at the recei)100.8 633 Q
-.165(ve)-.275 G -.605(r.).165 G(Data may no longer be useful for man)
100.8 646 Q 2.75(yr)-.165 G(easons:)-2.75 E 7.15<8349>100.8 662.6 S 2.75
(nat)-7.15 G(elephon)-2.75 E 2.75(yo)-.165 G 2.75(rs)-2.75 G
(treaming video session, data in a pack)-2.75 E
(et comprises a timeslice of a)-.11 E(continuous stream.)111.8 675.6 Q
(Once a timeslice has been played out, the ne)5.5 E
(xt timeslice is required)-.165 E(immediately)111.8 688.6 Q 5.5(.I)-.715
G 2.75(ft)-5.5 G(he data comprising that timeslice arri)-2.75 E -.165
(ve)-.275 G 2.75(sa).165 G 2.75(ts)-2.75 G
(ome later time, then it is no)-2.75 E(longer useful.)111.8 701.6 Q
(Such applications can cope with masking the ef)5.5 E
(fects of missing pack)-.275 E(ets to)-.11 E(some e)111.8 714.6 Q
(xtent, so when the sender transmits its ne)-.165 E(xt pack)-.165 E
(et, it is important for it to only)-.11 E(Flo)72 769 Q(yd/Handle)-.11 E
(y/K)-.165 E 235.76(ohler Section)-.385 F 2.75(2.1. [P)2.75 F(age 4])
-.165 E 0 Cg EP
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(December 2005)2.75 E(June 2005)116.406 E
(send data that has a good chance of arri)111.8 85 Q
(ving in time for its playout.)-.275 E 7.15<8349>100.8 101.6 S 2.75(na)
-7.15 G 2.75(ni)-2.75 G(nteracti)-2.75 E .33 -.165(ve g)-.275 H
(ame or VR session, position information is transient. If a datagram).11
E(containing position information is lost, resending the old position d\
oes not usually mak)111.8 114.6 Q(e)-.11 E(sense -- rather)111.8 127.6 Q
2.75(,e)-.44 G -.165(ve)-3.025 G
(ry position information datagram should contain the latest position)
.165 E(information.)111.8 140.6 Q(In a congestion-controlled \215o)100.8
157.2 Q 1.43 -.715(w, t)-.275 H(he allo).715 E(wed pack)-.275 E
(et sending rate depends on measured)-.11 E(netw)100.8 170.2 Q
(ork congestion. Thus, some control is gi)-.11 E -.165(ve)-.275 G 2.75
(nu).165 G 2.75(pt)-2.75 G 2.75(ot)-2.75 G
(he congestion control mechanism,)-2.75 E
(which determines precisely when pack)100.8 183.2 Q(ets can be sent. Ho)
-.11 E(we)-.275 E -.165(ve)-.275 G .88 -.44(r, a).165 H
(pplications could still).44 E(decide,)100.8 196.2 Q/F1 11
/Times-Italic@0 SF(at tr)2.75 E(ansmission time)-.165 E(,)-.11 E F0
(which information to put in a pack)2.75 E(et. TCP doesn')-.11 E 2.75
(ta)-.198 G(llo)-2.75 E(w)-.275 E(control o)100.8 209.2 Q -.165(ve)-.165
G 2.75(rt).165 G(his; these applications demand it.)-2.75 E
(Often, these applications \(especially g)100.8 225.8 Q
(ames and telephon)-.055 E 2.75(ya)-.165 G(pplications\) w)-2.75 E
(ork on v)-.11 E(ery short)-.165 E(playout timescales.)100.8 238.8 Q
(Whilst the)5.5 E 2.75(ya)-.165 G
(re usually able to adjust their transmission rate based on)-2.75 E
(congestion feedback, the)100.8 251.8 Q 2.75(yd)-.165 G 2.75(oh)-2.75 G
-2.475 -.22(av e)-2.75 H(constraints on ho)2.97 E 2.75(wt)-.275 G
(his adaptation can be performed so)-2.75 E
(that it has minimal impact on the quality of the session. Thus, the)
100.8 264.8 Q 2.75(yt)-.165 G(end to need some)-2.75 E(control o)100.8
277.8 Q -.165(ve)-.165 G 2.75(rt).165 G(he short-term dynamics of the c\
ongestion control algorithm, whilst being f)-2.75 E(air)-.11 E
(to other traf)100.8 290.8 Q
(\214c on medium timescales. This control includes, b)-.275 E
(ut is not limited to, some)-.22 E(in\215uence on which congestion cont\
rol algorithm should be used -- for e)100.8 303.8 Q(xample, TFRC)-.165 E
(rather than strict TCP-lik)100.8 316.8 Q 2.75(ec)-.11 G
(ongestion control. \(TCP-Friendly Rate Control, or TFRC, has)-2.75 E(b\
een standardized in the IETF as a congestion control mechanism that adj\
usts its sending)100.8 329.8 Q
(rate more smoothly than TCP does, while maintaining long-term f)100.8
342.8 Q(air bandwidth sharing)-.11 E(with TCP [RFC 3448].\))100.8 355.8
Q/F2 11/Times-Bold@0 SF(2.2.)72 385.4 Q/F3 13/Times-Bold@0 SF(Ov)5.5 E
(erhead)-.13 E F0(The applications we are concerned with often send com\
pressed data, or send frequent small)100.8 402 Q(pack)100.8 415 Q
(ets. F)-.11 E(or e)-.165 E(xample, when internet telephon)-.165 E 2.75
(yo)-.165 G 2.75(rs)-2.75 G(treaming media are used o)-2.75 E -.165(ve)
-.165 G 2.75(rl).165 G -.275(ow)-2.75 G(-).275 E(bandwidth modem links,\
 highly compressing the payload data is essential. F)100.8 428 Q
(or internet)-.165 E(telephon)100.8 441 Q 2.75(ya)-.165 G
(pplications and for g)-2.75 E(ames, the requirement is for lo)-.055 E
2.75(wd)-.275 G(elay)-2.75 E 2.75(,a)-.715 G(nd hence small)-2.75 E
(pack)100.8 454 Q(ets are sent frequently)-.11 E(.)-.715 E -.165(Fo)
100.8 470.6 S 2.75(re).165 G(xample, a telephon)-2.915 E 2.75(ya)-.165 G
(pplication sending a 5.6Kbps data stream b)-2.75 E(ut w)-.22 E(anting)
-.11 E(moderately lo)100.8 483.6 Q 2.75(wd)-.275 G(elay may send a pack)
-2.75 E(et e)-.11 E -.165(ve)-.275 G
(ry 20ms, sending only 14 data bytes in each).165 E(pack)100.8 496.6 Q
2.75(et. In)-.11 F(addition, 20 bytes is tak)2.75 E
(en up by the IP header)-.11 E 2.75(,w)-.44 G(ith additional bytes for)
-2.75 E(transport and/or application headers.)100.8 509.6 Q(Clearly)5.5
E 2.75(,f)-.715 G(or such an application it is desirable to ha)-2.75 E
-.165(ve)-.22 G 2.75(al)100.8 522.6 S .55 -.275(ow o)-2.75 H -.165(ve)
.11 G(rhead for the transport protocol header).165 E(.)-.605 E
(In some cases the correct solution w)100.8 539.2 Q
(ould be to use link-based pack)-.11 E(et header compression to)-.11 E
(compress the pack)100.8 552.2 Q
(et headers, although we cannot guarantee the a)-.11 E -.275(va)-.22 G
(ilability of such).275 E(compression schemes on an)100.8 565.2 Q 2.75
(yp)-.165 G(articular link.)-2.75 E
(The delay of data until after the completion of a handshak)100.8 581.8
Q 2.75(ea)-.11 G(lso represents potentially)-2.75 E(unnecessary o)100.8
594.8 Q -.165(ve)-.165 G(rhead. A ne).165 E 2.75(wp)-.275 G
(rotocol might therefore allo)-2.75 E 2.75(ws)-.275 G
(enders to include some data)-2.75 E(on their initial datagrams.)100.8
607.8 Q F2(2.3.)72 633.8 Q F3(Fir)5.5 E(ewall T)-.234 E(ra)-.962 E -.13
(ve)-.325 G(rsal).13 E F0(Applications requiring a \215o)100.8 650.4 Q
2.75(wo)-.275 G 2.75(fu)-2.75 G
(nreliable datagrams currently tend to use signalling)-2.75 E
(protocols such as R)100.8 663.4 Q(TSP)-.66 E 2.75(,S)-1.221 G
(IP and H.323 in conjunction with UDP for the data \215o)-2.75 E 4.18
-.715(w. T)-.275 H(he).715 E(initial setup request uses a signalling pr\
otocol to locate the correct remote end-system for)100.8 676.4 Q
(the data \215o)100.8 689.4 Q 1.43 -.715(w, s)-.275 H(ometimes being re\
directed or relayed to other machines, before the data).715 E<8d6f>100.8
702.4 Q 2.75(wi)-.275 G 2.75(se)-2.75 G(stablished.)-2.75 E(Flo)72 769 Q
(yd/Handle)-.11 E(y/K)-.165 E 235.76(ohler Section)-.385 F 2.75(2.3. [P)
2.75 F(age 5])-.165 E 0 Cg EP
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/F0 11/Times-Roman@0 SF(INTERNET)72 49 Q 66.092(-DRAFT Expires:)-1.012 F
(December 2005)2.75 E(June 2005)116.406 E(As UDP \215o)100.8 85 Q
(ws contain no e)-.275 E(xplicit setup and teardo)-.165 E
(wn, it is hard for \214re)-.275 E -.11(wa)-.275 G(lls to handle).11 E
(them correctly)100.8 98 Q 5.5(.T)-.715 G(ypically the \214re)-6.38 E
-.11(wa)-.275 G(ll needs to parse R).11 E(TSP)-.66 E 2.75(,S)-1.221 G
(IP and H.323 to obtain the)-2.75 E
(information necessary to open a hole in the \214re)100.8 111 Q -.11(wa)
-.275 G 2.75(ll. Alternati).11 F -.165(ve)-.275 G(ly).165 E 2.75(,f)
-.715 G(or bi-directional \215o)-2.75 E(ws,)-.275 E(the \214re)100.8 124
Q -.11(wa)-.275 G(ll can open a bi-directional hole if it recei).11 E
-.165(ve)-.275 G 2.75(saU).165 G(DP pack)-2.75 E(et from inside the)-.11
E(\214re)100.8 137 Q -.11(wa)-.275 G(ll, b).11 E
(ut in this case the \214re)-.22 E -.11(wa)-.275 G(ll can').11 E 2.75
(te)-.198 G(asily kno)-2.75 E 2.75(ww)-.275 G(hen to close the hole ag)
-2.75 E(ain.)-.055 E
(While we do not consider these to be major problems, the)100.8 153.6 Q
2.75(ya)-.165 G(re nonetheless issues that)-2.75 E
(application designers f)100.8 166.6 Q 2.75(ace. Currently)-.11 F
(streaming media players such as RealPlayer attempt)2.75 E
(UDP \214rst, and then switch to TCP if UDP is not successful.)100.8
179.6 Q(Streaming media o)5.5 E -.165(ve)-.165 G 2.75(rT).165 G(CP is)
-2.75 E(undesirable, and can result in the recei)100.8 192.6 Q -.165(ve)
-.275 G 2.75(rn).165 G(eeding to temporarily halt playout while it)-2.75
E("reb)100.8 205.6 Q(uf)-.22 E(fers" data.)-.275 E -.77(Te)5.5 G(lephon)
.77 E 2.75(ya)-.165 G(pplications don')-2.75 E 2.75(te)-.198 G -.165(ve)
-3.025 G 2.75(nh).165 G -2.475 -.22(av e)-2.75 H(this option.)2.97 E/F1
11/Times-Bold@0 SF(2.4.)72 231.6 Q/F2 13/Times-Bold@0 SF -.13(Pa)5.5 G
(rameter Negotiation).13 E F0(Dif)100.8 248.2 Q(ferent applications ha)
-.275 E .33 -.165(ve d)-.22 H(if).165 E
(ferent requirements for congestion control, which may map)-.275 E
(into dif)100.8 261.2 Q(ferent congestion feedback.)-.275 E
(Examples include ECN capability and desired)5.5 E(congestion control d\
ynamics \(the choice of congestion control algorithm and, therefore, th\
e)100.8 274.2 Q(form of feedback information required\).)100.8 287.2 Q
(Such parameters need to be reliably ne)5.5 E(gotiated)-.165 E
(before congestion control can function correctly)100.8 300.2 Q(.)-.715
E(While this ne)100.8 316.8 Q
(gotiation could be performed using signalling protocols such as SIP)
-.165 E 2.75(,R)-1.221 G(TSP)-3.41 E(and H.323, it w)100.8 329.8 Q
(ould be desirable to ha)-.11 E .33 -.165(ve a s)-.22 H
(ingle standard w).165 E(ay of ne)-.11 E(gotiating these)-.165 E
(transport parameters.)100.8 342.8 Q
(This is of particular importance with ECN, where sending ECN-)5.5 E
(mark)100.8 355.8 Q(ed pack)-.11 E(ets to a non-ECN-capable recei)-.11 E
-.165(ve)-.275 G 2.75(rc).165 G
(an cause signi\214cant congestion problems)-2.75 E(to other \215o)100.8
368.8 Q 2.75(ws. W)-.275 F 2.75(ed)-.88 G
(iscuss the ECN issue in more detail belo)-2.75 E -.715(w.)-.275 G F1
(3.)72 394.8 Q/F3 14/Times-Bold@0 SF(Solution Space f)5.5 E
(or Congestion Contr)-.35 E(ol of Unr)-.252 E(eliable Flo)-.252 E(ws)
-.14 E F0 1.76 -.88(We t)100.8 411.4 T(hus w).88 E(ant to pro)-.11 E
(vide congestion control for unreliable \215o)-.165 E(ws, pro)-.275 E
(viding both ECN and)-.165 E(the choice of dif)100.8 424.4 Q
(ferent forms of congestion control, and pro)-.275 E(viding moderate o)
-.165 E -.165(ve)-.165 G(rhead in).165 E(terms of pack)100.8 437.4 Q
(et size, state, and CPU processing.)-.11 E
(There are a number of options for)5.5 E(pro)100.8 450.4 Q
(viding end-to-end congestion control for the unicast traf)-.165 E
(\214c that currently uses UDP)-.275 E 2.75(,i)-1.221 G(n)-2.75 E
(terms of the layer that pro)100.8 463.4 Q
(vides the congestion control mechanism:)-.165 E 7.15<8343>100.8 480 S
(ongestion control abo)-7.15 E .33 -.165(ve U)-.165 H(DP).165 E(.)-1.221
E 7.15<8343>100.8 496.6 S(ongestion control belo)-7.15 E 2.75(wU)-.275 G
(DP)-2.75 E(.)-1.221 E 7.15<8343>100.8 513.2 S
(ongestion control at the transport layer in an alternati)-7.15 E .33
-.165(ve t)-.275 H 2.75(oU).165 G(DP)-2.75 E(.)-1.221 E 1.76 -.88(We e)
100.8 529.8 T(xplore these alternati).715 E -.165(ve)-.275 G 2.75(si)
.165 G 2.75(nt)-2.75 G(he sections belo)-2.75 E 4.18 -.715(w. T)-.275 H
(he concerns from the discussions).715 E(belo)100.8 542.8 Q 2.75(wh)
-.275 G -2.475 -.22(av e)-2.75 H(con)2.97 E(vinced us that the best w)
-.44 E(ay to pro)-.11 E(vide congestion control for unreliable)-.165 E
<8d6f>100.8 555.8 Q(ws is to pro)-.275 E
(vide congestion control at the transport layer)-.165 E 2.75(,a)-.44 G
2.75(sa)-2.75 G 2.75(na)-2.75 G(lternati)-2.75 E .33 -.165(ve t)-.275 H
2.75(ot).165 G(he use of)-2.75 E(UDP and TCP)100.8 568.8 Q(.)-1.221 E F1
(3.1.)72 594.8 Q F2(Pr)5.5 E -.13(ov)-.234 G(iding Congestion Contr).13
E(ol Abo)-.234 E .26 -.13(ve U)-.13 H(DP).13 E F0(One possibility w)
100.8 611.4 Q(ould be to pro)-.11 E
(vide congestion control at the application layer)-.165 E 2.75(,o)-.44 G
2.75(ra)-2.75 G 2.75(ts)-2.75 G(ome)-2.75 E(other layer abo)100.8 624.4
Q .33 -.165(ve U)-.165 H(DP).165 E 5.5(.T)-1.221 G(his w)-5.5 E
(ould allo)-.11 E 2.75(wt)-.275 G
(he congestion control mechanism to be closely)-2.75 E(inte)100.8 637.4
Q(grated with the application itself.)-.165 E F1 2.75(3.1.1. The)72
663.4 R(Burden on the A)2.75 E(pplication Designer)-.275 E F0 2.75(Ak)
100.8 680 S .33 -.165(ey d)-2.86 H(isadv).165 E(antage of pro)-.275 E
(viding congestion control abo)-.165 E .33 -.165(ve U)-.165 H
(DP is that it places an).165 E(unnecessary b)100.8 693 Q
(urden on the application-le)-.22 E -.165(ve)-.275 G 2.75(ld).165 G
(esigner)-2.75 E 2.75(,w)-.44 G(ho might be just as happ)-2.75 E 2.75
(yt)-.11 G 2.75(ou)-2.75 G(se)-2.75 E(the congestion control pro)100.8
706 Q(vided by a lo)-.165 E(wer layer)-.275 E 5.5(.I)-.605 G 2.75(ft)
-5.5 G(he application can rely on a lo)-2.75 E(wer)-.275 E
(layer that gi)100.8 719 Q -.165(ve)-.275 G 2.75(sac).165 G
(hoice between TCP-lik)-2.75 E 2.75(eo)-.11 G 2.75(rT)-2.75 G(FRC-lik)
-2.75 E 2.75(ec)-.11 G(ongestion control, and that)-2.75 E(Flo)72 769 Q
(yd/Handle)-.11 E(y/K)-.165 E 227.51(ohler Section)-.385 F 2.75
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(December 2005)2.75 E(June 2005)116.406 E(of)100.8 85 Q
(fers ECN, then this might be highly satisf)-.275 E(actory to man)-.11 E
2.75(ya)-.165 G(pplication designers.)-2.75 E(The long history of deb)
100.8 101.6 Q(ugging TCP implementations [RFC 2525, TBIT] mak)-.22 E
(es the)-.11 E(dif)100.8 114.6 Q
(\214culties in implementing end-to-end congestion control ab)-.275 E
(undantly clear)-.22 E 5.5(.I)-.605 G 2.75(ti)-5.5 G 2.75(sc)-2.75 G
(learly)-2.75 E(more rob)100.8 127.6 Q
(ust for congestion control to be pro)-.22 E
(vided for the application by a lo)-.165 E(wer layer)-.275 E 5.5(.I)
-.605 G(n)-5.5 E(rare cases there might be compelling reasons for the c\
ongestion control mechanism to be)100.8 140.6 Q
(implemented in the application itself, b)100.8 153.6 Q(ut we do not e)
-.22 E(xpect this to be the general case.)-.165 E -.165(Fo)5.5 G(r).165
E -.165(ex)100.8 166.6 S(ample, applications that use R).165 E(TP o)-.66
E -.165(ve)-.165 G 2.75(rU).165 G(DP might be just as happ)-2.75 E 2.75
(yi)-.11 G 2.75(fR)-2.75 G(TP itself)-3.41 E
(implemented end-to-end congestion control.)100.8 179.6 Q
(\(See Section 3.3.3 for more discussion of)5.5 E -.66(RT)100.8 192.6 S
-1.221(P.).66 G(\))1.221 E(In addition to congestion control issues, we\
 also note the problems with \214re)100.8 209.2 Q -.11(wa)-.275 G
(ll tra).11 E -.165(ve)-.22 G(rsal).165 E(and parameter ne)100.8 222.2 Q
(gotiation discussed in sections 2.3 and 2.4. Implementing on top of UD\
P)-.165 E
(requires that the application designer also address these issues.)100.8
235.2 Q/F1 11/Times-Bold@0 SF 2.75(3.1.2. Dif\214culties)72 261.2 R
(with ECN)2.75 E F0 2.75(As)100.8 277.8 S(econd problem of pro)-2.75 E
(viding congestion control abo)-.165 E .33 -.165(ve U)-.165 H
(DP is that it w).165 E(ould require)-.11 E(either gi)100.8 290.8 Q
(ving up the use of ECN, or gi)-.275 E
(ving the application direct control o)-.275 E -.165(ve)-.165 G 2.75(rs)
.165 G(etting and)-2.75 E(reading the ECN \214eld in the IP header)100.8
303.8 Q 5.5(.G)-.605 G -.275(iv)-5.5 G(ing up the use of ECN w).275 E
(ould be problematic,)-.11 E
(since ECN can be particularly useful for unreliable \215o)100.8 316.8 Q
(ws, where a dropped pack)-.275 E(et will not)-.11 E
(be retransmitted by the data sender)100.8 329.8 Q(.)-.605 E -.44(Wi)
100.8 346.4 S(th the de).44 E -.165(ve)-.275 G
(lopment of the ECN nonce, ECN can also be useful e).165 E -.165(ve)
-.275 G 2.75(ni).165 G 2.75(nt)-2.75 G(he absence of)-2.75 E
(ECN support from the netw)100.8 359.4 Q 2.75(ork. The)-.11 F
(data sender can use the ECN nonce, along with)2.75 E
(feedback from the data recei)100.8 372.4 Q -.165(ve)-.275 G .88 -.44
(r, t).165 H 2.75(ov).44 G(erify that the data recei)-2.915 E -.165(ve)
-.275 G 2.75(ri).165 G 2.75(sc)-2.75 G(orrectly reporting all lost)-2.75
E(pack)100.8 385.4 Q 2.75(ets. This)-.11 F(use of ECN can be particular\
ly useful for an application using unreliable)2.75 E(deli)100.8 398.4 Q
-.165(ve)-.275 G(ry).165 E 2.75(,w)-.715 G(here the recei)-2.75 E -.165
(ve)-.275 G 2.75(rm).165 G(ight otherwise ha)-2.75 E .33 -.165(ve l)-.22
H(ittle incenti).165 E .33 -.165(ve t)-.275 H 2.75(or).165 G
(eport lost pack)-2.75 E(ets.)-.11 E(In order to allo)100.8 415 Q 2.75
(wt)-.275 G(he use of ECN by a layer abo)-2.75 E .33 -.165(ve U)-.165 H
(DP).165 E 2.75(,t)-1.221 G(he UDP sock)-2.75 E(et w)-.11 E(ould ha)-.11
E .33 -.165(ve t)-.22 H(o).165 E(allo)100.8 428 Q 2.75(wt)-.275 G
(he application to control the ECN \214eld in the IP header)-2.75 E 5.5
(.I)-.605 G 2.75(np)-5.5 G(articular)-2.75 E 2.75(,t)-.44 G(he UDP)-2.75
E(sock)100.8 441 Q(et w)-.11 E(ould ha)-.11 E .33 -.165(ve t)-.22 H 2.75
(oa).165 G(llo)-2.75 E 2.75(wt)-.275 G
(he application to specify whether or not the ECN-Capable)-2.75 E -.385
(Tr)100.8 454 S(ansport \(ECT\) codepoints should be set in the ECN \
\214eld of the IP header).385 E(.)-.605 E(The ECN contract is that send\
ers who set the ECT codepoint must respond to CE)100.8 470.6 Q
(codepoints by reducing their sending rates.)100.8 483.6 Q
(Therefore, the ECT codepoint can only safely)5.5 E(be set in the pack)
100.8 496.6 Q(et header of a UDP pack)-.11 E(et if the follo)-.11 E
(wing is guaranteed:)-.275 E 7.15<8349>100.8 513.2 S 2.75(ft)-7.15 G
(he Congestion Experienced \(CE\) codepoint is set by a router)-2.75 E
2.75(,t)-.44 G(he recei)-2.75 E(ving IP layer)-.275 E
(will pass the CE status to the UDP layer)111.8 526.2 Q 2.75(,w)-.44 G
(hich will pass it to the recei)-2.75 E(ving application)-.275 E
(at the data recei)111.8 539.2 Q -.165(ve)-.275 G .88 -.44(r, a).165 H
(nd:).44 E 7.15<8355>100.8 555.8 S(pon recei)-7.15 E(ving a pack)-.275 E
(et that had the CE codepoint set, the recei)-.11 E
(ving application will)-.275 E(tak)111.8 568.8 Q 2.75(et)-.11 G(he appr\
opriate congestion control action, such as informing the data sender)
-2.75 E(.)-.605 E(Ho)100.8 585.4 Q(we)-.275 E -.165(ve)-.275 G .88 -.44
(r, t).165 H(he UDP implementation at the data sender has no w).44 E
(ay of kno)-.11 E(wing if the UDP)-.275 E
(implementation at the data recei)100.8 598.4 Q -.165(ve)-.275 G 2.75
(rh).165 G(as been upgraded to pass a CE status up to the)-2.75 E(recei)
100.8 611.4 Q(ving application, let alone whether or not the applicatio\
n will use the conformant end-)-.275 E
(to-end congestion control that goes along with use of ECN.)100.8 624.4
Q(In the absence of the widespread deplo)100.8 641 Q
(yment of mechanisms in routers to detect \215o)-.11 E(ws that)-.275 E
(are not using conformant congestion control, allo)100.8 654 Q
(wing applications arbitrary control of the)-.275 E
(ECT codepoints for UDP pack)100.8 667 Q(ets w)-.11 E(ould seem lik)-.11
E 2.75(ea)-.11 G 2.75(nu)-2.75 G(nnecessary opportunity for)-2.75 E
(applications to use ECN while e)100.8 680 Q -.275(va)-.275 G
(ding the use of end-to-end congestion control.).275 E(Thus,)5.5 E
(there is an inherent "chick)100.8 693 Q(en-and-e)-.11 E
(gg" problem of whether \214rst to deplo)-.165 E 2.75(yp)-.11 G(olicing)
-2.75 E(mechanisms in routers, or \214rst to enable the use of ECN by U\
DP \215o)100.8 706 Q 2.75(ws. W)-.275 F(ithout the)-.44 E
(policing mechanisms in routers, we w)100.8 719 Q
(ould not advise adding ECN-capability to UDP)-.11 E(Flo)72 769 Q
(yd/Handle)-.11 E(y/K)-.165 E 227.51(ohler Section)-.385 F 2.75
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(December 2005)2.75 E(June 2005)116.406 E(sock)100.8 85 Q
(ets at this time.)-.11 E(In the absence of more \214ne-grained mechani\
sms for dealing with a period of sustained)100.8 101.6 Q
(congestion, one possibility w)100.8 114.6 Q
(ould be for routers to discontinue using ECN with UDP)-.11 E(pack)100.8
127.6 Q(ets during the congested period, and to use ECN only with TCP o\
r DCCP pack)-.11 E(ets.)-.11 E(This w)100.8 140.6 Q
(ould be a reasonable response, for e)-.11 E
(xample, if TCP or DCCP \215o)-.165 E(ws were found to be)-.275 E
(more lik)100.8 153.6 Q(ely to be using conformant end-to-end congestio\
n control than were UDP \215o)-.11 E 2.75(ws. If)-.275 F
(routers were to adopt such a polic)100.8 166.6 Q 1.43 -.715(y, t)-.165
H(hen DCCP \215o).715 E(ws could be more lik)-.275 E(ely to recei)-.11 E
.33 -.165(ve t)-.275 H(he).165 E
(bene\214ts of ECN in times of congestion than w)100.8 179.6 Q
(ould UDP \215o)-.11 E(ws.)-.275 E/F1 11/Times-Bold@0 SF 2.75
(3.1.3. The)72 205.6 R(Ev)2.75 E(asion of Congestion Contr)-.11 E(ol)
-.198 E F0 2.75(At)100.8 222.2 S(hird problem of pro)-2.75 E
(viding congestion control abo)-.165 E .33 -.165(ve U)-.165 H
(DP is that relying on congestion).165 E(control at the application le)
100.8 235.2 Q -.165(ve)-.275 G 2.75(lm).165 G(ak)-2.75 E(es it some)-.11
E(what easier for some users to e)-.275 E -.275(va)-.275 G(de end-to-)
.275 E(end congestion control.)100.8 248.2 Q 1.76 -.88(We d)5.5 H 2.75
(on).88 G(ot claim that a transport protocol such as DCCP w)-2.75 E
(ould)-.11 E(al)100.8 261.2 Q -.11(wa)-.11 G(ys be implemented in the k)
.11 E(ernel, and do not attempt to e)-.11 E -.275(va)-.275 G
(luate the relati).275 E .33 -.165(ve d)-.275 H(if).165 E(\214culty)
-.275 E(of modifying code inside the k)100.8 274.2 Q
(ernel vs. outside the k)-.11 E(ernel in an)-.11 E 2.75(yc)-.165 G 2.75
(ase. Ho)-2.75 F(we)-.275 E -.165(ve)-.275 G .88 -.44(r, w).165 H(e).44
E(belie)100.8 287.2 Q .33 -.165(ve t)-.275 H
(hat putting the congestion control at the transport le).165 E -.165(ve)
-.275 G 2.75(lr).165 G(ather than at the)-2.75 E(application le)100.8
300.2 Q -.165(ve)-.275 G 2.75(lm).165 G(ak)-2.75 E
(es it just slightly less lik)-.11 E
(ely that users will go to the trouble of)-.11 E
(modifying the code in order to a)100.8 313.2 Q -.22(vo)-.22 G
(id using end-to-end congestion control.).22 E F1(3.2.)72 339.2 Q/F2 13
/Times-Bold@0 SF(Pr)5.5 E -.13(ov)-.234 G(iding Congestion Contr).13 E
(ol Belo)-.234 E 3.25(wU)-.13 G(DP)-3.25 E F0(Instead of pro)100.8 355.8
Q(viding congestion control abo)-.165 E .33 -.165(ve U)-.165 H(DP).165 E
2.75(,as)-1.221 G(econd possibility w)-2.75 E(ould be to)-.11 E(pro)
100.8 368.8 Q
(vide congestion control for unreliable applications at a layer belo)
-.165 E 2.75(wU)-.275 G(DP)-2.75 E 2.75(,w)-1.221 G(ith)-2.75 E
(applications using UDP as their transport protocol.)100.8 381.8 Q(Gi)
5.5 E -.165(ve)-.275 G 2.75(nt).165 G(hat UDP does not itself pro)-2.75
E(vide)-.165 E(sequence numbers or congestion feedback, there are tw)
100.8 394.8 Q 2.75(op)-.11 G(ossible forms for this congestion)-2.75 E
(feedback:)100.8 407.8 Q 7.15<8328>100.8 424.4 S
(1\) Feedback at the application: The application abo)-7.15 E .33 -.165
(ve U)-.165 H(DP could pro).165 E(vide sequence)-.165 E
(numbers and feedback to the sender)111.8 437.4 Q 2.75(,w)-.44 G(hich w)
-2.75 E(ould then communicate loss information to)-.11 E
(the congestion control mechanism.)111.8 450.4 Q
(This is the approach currently standardized by the)5.5 E
(Congestion Manager [RFC 3124].)111.8 463.4 Q 7.15<8328>100.8 480 S
(2\) Feedback at the layer belo)-7.15 E 2.75(wU)-.275 G
(DP: The application could use UDP)-2.75 E 2.75(,a)-1.221 G
(nd a protocol)-2.75 E
(could be implemented using a shim header between IP and UDP to pro)
111.8 493 Q(vide sequence)-.165 E(number information for data pack)111.8
506 Q(ets and return feedback to the data sender)-.11 E 5.5(.T)-.605 G
(he)-5.5 E
(original proposal for the Congestion Manager [Bala99] suggested pro)
111.8 519 Q(viding this layer)-.165 E(for applications that did not ha)
111.8 532 Q .33 -.165(ve t)-.22 H(heir o).165 E
(wn feedback about dropped pack)-.275 E(ets.)-.11 E 1.76 -.88(We d)100.8
548.6 T(iscuss these tw).88 E 2.75(oc)-.11 G(ases separately belo)-2.75
E -.715(w.)-.275 G F1 2.75(3.2.1. Case)72 574.6 R(1: Congestion F)2.75 E
(eedback at the A)-.275 E(pplication)-.275 E F0
(In this case, the application pro)100.8 591.2 Q
(vides sequence numbers and congestion feedback abo)-.165 E -.165(ve)
-.165 G(UDP)100.8 604.2 Q 2.75(,b)-1.221 G
(ut communicates that feedback to a congestion manager belo)-2.97 E 2.75
(wU)-.275 G(DP)-2.75 E 2.75(,w)-1.221 G(hich)-2.75 E(re)100.8 617.2 Q
(gulates when pack)-.165 E(ets can be sent.)-.11 E(This approach suf)5.5
E(fers from most of the problems)-.275 E(described in Section 3.1, name\
ly forcing the application designer to rein)100.8 630.2 Q -.165(ve)-.44
G(nt the wheel).165 E(each time for pack)100.8 643.2 Q
(et formats and parameter ne)-.11 E
(gotiation, and problems with ECN usage,)-.165 E(\214re)100.8 656.2 Q
-.11(wa)-.275 G(lls and e).11 E -.275(va)-.275 G(sion.).275 E(It w)100.8
672.8 Q(ould a)-.11 E -.22(vo)-.22 G
(id the application writer needing to implement the control part of the)
.22 E(congestion control mechanism, b)100.8 685.8 Q(ut it is unclear ho)
-.22 E 2.75(we)-.275 G(asily multiple congestion control)-2.75 E
(algorithms \(such as recei)100.8 698.8 Q -.165(ve)-.275 G -.22(r-).165
G(based TFRC\) can be supported, gi).22 E -.165(ve)-.275 G 2.75(nt).165
G(hat the form of)-2.75 E(congestion feedback usually needs to be close\
ly coupled to the congestion control)100.8 711.8 Q
(algorithm being used.)100.8 724.8 Q
(Thus, this design limits the choice of congestion control)5.5 E(Flo)72
769 Q(yd/Handle)-.11 E(y/K)-.165 E 227.51(ohler Section)-.385 F 2.75
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(December 2005)2.75 E(June 2005)116.406 E(mechanisms a)100.8 85 Q -.275
(va)-.22 G(ilable to applications while simultaneously b).275 E
(urdening the applications with)-.22 E
(implementations of congestion feedback.)100.8 98 Q/F1 11/Times-Bold@0
SF 2.75(3.2.2. Case)72 124 R(2: Congestion F)2.75 E(eedback at a Lay)
-.275 E(er belo)-.11 E 2.75(wU)-.11 G(DP)-2.75 E F0(Pro)100.8 140.6 Q
(viding feedback at a layer belo)-.165 E 2.75(wU)-.275 G(DP w)-2.75 E
(ould require an additional pack)-.11 E(et header belo)-.11 E(w)-.275 E
(UDP to carry sequence numbers in addition to the eight-byte header for\
 UDP itself.)100.8 153.6 Q(Unless)5.5 E
(this header were an IP option \(which is lik)100.8 166.6 Q
(ely to cause problems for man)-.11 E 2.75(yI)-.165 G(Pv4 routers\))
-2.75 E(then its presence w)100.8 179.6 Q
(ould need to be indicated using a dif)-.11 E(ferent IP protocol v)-.275
E(alue from UDP)-.275 E(.)-1.221 E(Thus, the pack)100.8 192.6 Q(ets w)
-.11 E(ould no longer look lik)-.11 E 2.75(eU)-.11 G
(DP on the wire, and the modi\214ed protocol)-2.75 E -.11(wo)100.8 205.6
S(uld f).11 E(ace deplo)-.11 E
(yment challenges similar to those of an entirely ne)-.11 E 2.75(wp)
-.275 G(rotocol.)-2.75 E 1.76 -.88(To u)100.8 222.2 T
(se congestion feedback at a layer belo).88 E 2.75(wU)-.275 G
(DP most ef)-2.75 E(fecti)-.275 E -.165(ve)-.275 G(ly).165 E 2.75(,t)
-.715 G(he semantics of the)-2.75 E(UDP sock)100.8 235.2 Q
(et API \(Application Programming Interf)-.11 E(ace\) w)-.11 E
(ould also need changing, both to)-.11 E
(support a late decision on what to send, and to pro)100.8 248.2 Q
(vide access to the sequence numbers to)-.165 E -.22(avo)100.8 261.2 S
(id the application needing to duplicate them for its o).22 E
(wn purposes.)-.275 E(Thus, the sock)5.5 E(et API)-.11 E -.11(wo)100.8
274.2 S(uld no longer look lik).11 E 2.75(eU)-.11 G(DP to end hosts.)
-2.75 E(This w)5.5 E(ould ef)-.11 E(fecti)-.275 E -.165(ve)-.275 G
(ly be a ne).165 E 2.75(wt)-.275 G(ransport)-2.75 E(protocol.)100.8
287.2 Q(Gi)100.8 303.8 Q -.165(ve)-.275 G 2.75(nt).165 G
(hese complications, it seems cleaner to actually design a ne)-2.75 E
2.75(wt)-.275 G(ransport protocol,)-2.75 E(which also allo)100.8 316.8 Q
(ws us to address the issues of \214re)-.275 E -.11(wa)-.275 G(ll tra)
.11 E -.165(ve)-.22 G(rsal, \215o).165 E 2.75(ws)-.275 G
(etup, and parameter)-2.75 E(ne)100.8 329.8 Q 2.75(gotiation. W)-.165 F
2.75(en)-.88 G(ote that an)-2.75 E 2.75(yn)-.165 G .55 -.275(ew t)-2.75
H(ransport protocol could also use a Congestion Manager).275 E
(approach to share congestion state between \215o)100.8 342.8 Q
(ws using the same congestion control)-.275 E
(algorithm, if this were deemed to be desirable.)100.8 355.8 Q F1(3.3.)
72 381.8 Q/F2 13/Times-Bold@0 SF(Pr)5.5 E -.13(ov)-.234 G
(iding Congestion Contr).13 E(ol at the T)-.234 E(ransport Lay)-.962 E
(er)-.13 E F0(The concerns from the discussions abo)100.8 398.4 Q .33
-.165(ve h)-.165 H -2.475 -.22(av e).165 H(con)2.97 E
(vinced us that the best w)-.44 E(ay to pro)-.11 E(vide)-.165 E
(congestion control to applications that currently use UDP is to pro)
100.8 411.4 Q(vide congestion control)-.165 E(at the transport layer)
100.8 424.4 Q 2.75(,i)-.44 G 2.75(nat)-2.75 G
(ransport protocol used as an alternati)-2.75 E .33 -.165(ve t)-.275 H
2.75(oU).165 G(DP)-2.75 E 5.5(.O)-1.221 G(ne adv)-5.5 E(antage)-.275 E
(of pro)100.8 437.4 Q(viding end-to-end congestion control in an unreli\
able transport protocol is that it)-.165 E(could be used easily by a wi\
de range of the applications that currently use UDP)100.8 450.4 Q 2.75
(,w)-1.221 G(ith)-2.75 E(minimal changes to the application itself.)
100.8 463.4 Q(The application itself w)5.5 E(ould not ha)-.11 E .33
-.165(ve t)-.22 H 2.75(op).165 G(ro)-2.75 E(vide)-.165 E
(the congestion control mechanism, or e)100.8 476.4 Q -.165(ve)-.275 G
2.75(nt).165 G(he feedback from the data recei)-2.75 E -.165(ve)-.275 G
2.75(rt).165 G 2.75(ot)-2.75 G(he data)-2.75 E
(sender about lost or mark)100.8 489.4 Q(ed pack)-.11 E(ets.)-.11 E(The\
 question then arises of whether to adapt TCP for use by unreliable app\
lications, to use)100.8 506 Q(an unreliable v)100.8 519 Q
(ariant of SCTP or a v)-.275 E(ersion of R)-.165 E(TP with b)-.66 E
(uilt-in congestion control, or to)-.22 E(design a ne)100.8 532 Q 2.75
(wt)-.275 G(ransport protocol.)-2.75 E(As we ar)100.8 548.6 Q(gue belo)
-.198 E 1.43 -.715(w, t)-.275 H(he desire for minimal o).715 E -.165(ve)
-.165 G(rhead results in the design decision to use a).165 E
(transport protocol containing only the minimal necessary functionality)
100.8 561.6 Q 2.75(,a)-.715 G(nd to lea)-2.75 E .33 -.165(ve o)-.22 H
(ther).165 E(functionality such as reliability)100.8 574.6 Q 2.75(,s)
-.715 G(emi-reliability)-2.75 E 2.75(,o)-.715 G 2.75(rF)-2.75 G(orw)
-2.915 E(ard Error Correction \(FEC\) to be)-.11 E(layered on top.)100.8
587.6 Q F1 2.75(3.3.1. Modifying)72 613.6 R(TCP?)2.75 E F0
(One alternati)100.8 630.2 Q .33 -.165(ve m)-.275 H
(ight be to create an unreliable v).165 E(ariant of TCP)-.275 E 2.75(,w)
-1.221 G(ith the reliability layered)-2.75 E
(on top for applications desiring reliable deli)100.8 643.2 Q -.165(ve)
-.275 G(ry).165 E 5.5(.H)-.715 G -.275(ow)-5.5 G -2.365 -.275(ev e).275
H .88 -.44(r, o).275 H(ur requirement is not simply).44 E
(for TCP minus the in-order reliable deli)100.8 656.2 Q -.165(ve)-.275 G
(ry).165 E 2.75(,b)-.715 G(ut also for the application to be able to)
-2.97 E(choose congestion control algorithms.)100.8 669.2 Q(TCP')5.5 E
2.75(sf)-.605 G(eedback mechanism w)-2.75 E(orks well for TCP-)-.11 E
(lik)100.8 682.2 Q 2.75(ec)-.11 G(ongestion control, b)-2.75 E
(ut is inappropriate \(or at the v)-.22 E(ery least, inef)-.165 E
(\214cient\) for TFRC.)-.275 E(In)5.5 E
(addition, TCP sequence numbers are in bytes, not datagrams. This w)
100.8 695.2 Q(ould complicate both)-.11 E(congestion feedback and an)
100.8 708.2 Q 2.75(ya)-.165 G(ttempt to allo)-2.75 E 2.75(wt)-.275 G
(he application to decide, at transmission)-2.75 E
(time, what information should go into a pack)100.8 721.2 Q 2.75
(et. Finally)-.11 F 2.75(,t)-.715 G(here is the issue of whether a)-2.75
E(Flo)72 769 Q(yd/Handle)-.11 E(y/K)-.165 E 227.51(ohler Section)-.385 F
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(TCP using the same IP protocol number could ha)100.8 98 Q .33 -.165
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(middlebox)100.8 111 Q(es already deplo)-.165 E(yed in the netw)-.11 E
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(w)-.275 E(small, tak)100.8 153.6 Q
(es an inordinate amount of time to standardize and deplo)-.11 E 1.43
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166.6 Q(\214culty of getting TCP)-.275 E(implementations right.)100.8
179.6 Q/F1 11/Times-Bold@0 SF 2.75(3.3.2. Unr)72 205.6 R(eliable V)-.198
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Q -.165(ve)-.275 G(ry).165 E 5.5(.H)-.715 G -.275(ow)-5.5 G -2.365 -.275
(ev e).275 H .88 -.44(r, e).275 H
(xplicit support for multiple streams o).275 E -.165(ve)-.165 G 2.75
(ras).165 G(ingle \215o)-2.75 E 2.75(wa)-.275 G 2.75(tt)-2.75 G
(he transport)-2.75 E(layer is not necessary for an unreliable transpor\
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(necessity allo)100.8 274.2 Q(ws out-of-order deli)-.275 E -.165(ve)
-.275 G(ry).165 E 5.5(.B)-.715 G
(ecause an unreliable transport does not need)-5.5 E
(streams support, applications should not ha)100.8 287.2 Q .33 -.165
(ve t)-.22 H 2.75(op).165 G(ay the penalties in terms of increased)-2.75
E(header size that accompan)100.8 300.2 Q 2.75(yt)-.165 G
(he use of streams in SCTP)-2.75 E(.)-1.221 E
(The basic underlying structure of the SCTP pack)100.8 316.8 Q
(et, of a common SCTP header follo)-.11 E(wed by)-.275 E
(chunks for data, SA)100.8 329.8 Q(CK information, and so on, is moti)
-.44 E -.275(va)-.275 G(ted by SCTP').275 E 2.75(sg)-.605 G(oal of)-2.75
E(accommodating multiple streams, Ho)100.8 342.8 Q(we)-.275 E -.165(ve)
-.275 G .88 -.44(r, t).165 H(his use of chunks comes at the cost of an)
.44 E(increased header size for pack)100.8 355.8 Q
(ets, as each chunk must be aligned on 32-bit boundaries, and)-.11 E
(therefore requires a \214x)100.8 368.8 Q(ed-size 4-byte chunk header)
-.165 E 5.5(.F)-.605 G(or e)-5.665 E(xample, for a connection using)
-.165 E(ECN, SCTP includes separate control chunks for the Explicit Con\
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(indo)-.44 E 2.75(wR)-.275 G
(educed functions, with the ECNE and CWR chunks each)-2.75 E
(requiring 8 bytes.)100.8 407.8 Q(As another e)5.5 E
(xample, the common header includes a 4-byte v)-.165 E(eri\214cation)
-.165 E(tag to v)100.8 420.8 Q(alidate the sender)-.275 E 5.5(.T)-.605 G
(he issue of a minimal header size is just one of the w)-5.5 E(ays that)
-.11 E(the underlying frame)100.8 433.8 Q -.11(wo)-.275 G
(rk of SCTP does not \214t the needs of an unreliable transport as).11 E
(outlined in this document.)100.8 446.8 Q
(As a second concern, SCTP as currently speci\214ed uses TCP-lik)100.8
463.4 Q 2.75(ec)-.11 G(ongestion control, and)-2.75 E(does not pro)100.8
476.4 Q(vide support for alternati)-.165 E .33 -.165(ve c)-.275 H
(ongestion control algorithms such as TFRC that).165 E -.11(wo)100.8
489.4 S(uld be more attracti).11 E .33 -.165(ve t)-.275 H 2.75(os).165 G
(ome of the applications currently using UDP \215o)-2.75 E 2.75
(ws. Thus,)-.275 F(the)2.75 E(current v)100.8 502.4 Q(ersion of SCTP w)
-.165 E(ould not meet the requirements for a choice between forms of)
-.11 E(end-to-end congestion control.)100.8 515.4 Q
(One could suggest adding support for alternati)100.8 532 Q .33 -.165
(ve c)-.275 H(ongestion control mechanisms as an).165 E(option to SCTP)
100.8 545 Q 2.75(,a)-1.221 G(nd adding a fully-unreliable v)-2.75 E
(ariant that does not include the mechanisms)-.275 E
(for multiple streams.)100.8 558 Q(Ho)5.5 E(we)-.275 E -.165(ve)-.275 G
.88 -.44(r, t).165 H(hat path leads one to a choice between a `).44 E
(`kitchen sink')-.814 E(')-.814 E(protocol that tries to include option\
s to accommodate all applications \(i.e., the modi\214ed)100.8 571 Q
-.165(ve)100.8 584 S
(rsion of SCTP\), and a clean and minimal protocol that pro).165 E
(vides only end-to-end)-.165 E(congestion control and an)100.8 597 Q
2.75(yo)-.165 G(ther mechanisms that cannot be pro)-2.75 E
(vided in a higher layer)-.165 E(.)-.605 E 1.76 -.88(We w)100.8 610 T
(ould ar).77 E(gue ag)-.198 E(ainst the `)-.055 E(`kitchen sink')-.814 E
2.75('a)-.814 G(pproach in this case; aside from the higher)-2.75 E
(header o)100.8 623 Q -.165(ve)-.165 G(rhead, a `).165 E
(`kitchen sink ')-.814 E 2.75('p)-.814 G(rotocol seems more lik)-2.75 E
(ely to ha)-.11 E .33 -.165(ve u)-.22 H(nforseen).165 E
(interactions and dif)100.8 636 Q
(\214culties in making modi\214cations do)-.275 E(wn the road.)-.275 E(\
Applications that desire limited retransmission with multi-stream suppo\
rt, or that desire)100.8 652.6 Q
(multi-homing support, might be good candidates for a semi-reliable v)
100.8 665.6 Q(ersion of SCTP such)-.165 E(as U-SCTP)100.8 678.6 Q 5.5
(.H)-1.221 G -.275(ow)-5.5 G -2.365 -.275(ev e).275 H .88 -.44(r, w).275
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(ariants of SCTP are not suitable for fully-unreliable)-.275 E
(applications, or for the b)100.8 691.6 Q
(ulk of applications that today use UDP)-.22 E 5.5(.W)-1.221 G 2.75(ew)
-6.38 G(ould ar)-2.86 E(gue that)-.198 E(instead, an additional transpo\
rt protocol is needed for unreliable transfer)100.8 704.6 Q(.)-.605 E
(Flo)72 769 Q(yd/Handle)-.11 E(y/K)-.165 E 222.01(ohler Section)-.385 F
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-.165(ve)-.275 G(ral of our tar).165 E(get applications currently use R)
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(DP to transfer their).165 E(data. Wh)100.8 114.6 Q 2.75(yn)-.055 G
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144.2 Q 2.75(,u)-.44 G(ser)-2.75 E(-le)-.22 E -.165(ve)-.275 G 2.75(lR)
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(\214cult or)-.275 E(undesirable to allo)100.8 170.2 Q 2.75(wt)-.275 G
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G(rograms.)-2.75 E -.275(Ke)100.8 186.8 S(rnel implementations of R).275
E(TP w)-.66 E(ould not suf)-.11 E
(fer from this problem. In the end, the)-.275 E(ar)100.8 199.8 Q
(gument ag)-.198 E(ainst modifying R)-.055 E(TP is the same as that ag)
-.66 E(ainst modifying SCTP: Some)-.055 E(applications, such as the e)
100.8 212.8 Q(xport of \215o)-.165 E 2.75(wi)-.275 G
(nformation from routers, need congestion control)-2.75 E -.22(bu)100.8
225.8 S 2.75(td).22 G(on')-2.75 E 2.75(tn)-.198 G(eed much of R)-2.75 E
(TP')-.66 E 2.75(sf)-.605 G(unctionality)-2.75 E 2.75(.F)-.715 G
(rom these applications' point of vie)-2.75 E 1.43 -.715(w, R)-.275 H
(TP).055 E -.11(wo)100.8 238.8 S(uld induce unnecessary o).11 E -.165
(ve)-.165 G(rhead. Ag).165 E(ain, we w)-.055 E(ould ar)-.11 E
(gue for a clean and minimal)-.198 E
(protocol focused on end-to-end congestion control.)100.8 251.8 Q -.66
(RT)100.8 268.4 S 2.75(Pw).66 G(ould commonly be used as a layer abo)
-2.86 E .33 -.165(ve a)-.165 H .33 -.165(ny n).165 H .55 -.275(ew t).165
H(ransport protocol, ho).275 E(we)-.275 E -.165(ve)-.275 G 1.21 -.605
(r. T).165 H(he).605 E(design of that ne)100.8 281.4 Q 2.75(wt)-.275 G
(ransport protocol should tak)-2.75 E 2.75(et)-.11 G
(his into account, either by a)-2.75 E -.22(vo)-.22 G(iding).22 E
(undue duplication of information a)100.8 294.4 Q -.275(va)-.22 G
(ilable in the R).275 E(TP header)-.66 E 2.75(,o)-.44 G 2.75(rb)-2.75 G
2.75(ys)-2.75 G(uggesting)-2.75 E(modi\214cations to R)100.8 307.4 Q(TP)
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-.66 E -.165(ve)-.165 G 2.75(sa).165 G .33 -.165(ny \214)-2.75 H
(elds redundant with).165 E(the ne)100.8 320.4 Q 2.75(wp)-.275 G
(rotocol')-2.75 E 2.75(sh)-.605 G(eaders.)-2.75 E F1 2.75
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-.814 E(otocol)-.198 E F0(In the \214rst half of this document we ha)
100.8 363 Q .33 -.165(ve a)-.22 H -.198(rg).165 G(ued for pro).198 E
(viding congestion control at the)-.165 E
(transport layer as an alternati)100.8 376 Q .33 -.165(ve t)-.275 H 2.75
(oU).165 G(DP)-2.75 E 2.75(,i)-1.221 G
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389 Q .33 -.165(ve o)-.165 H 2.75(rb).165 G(elo)-2.75 E 2.75(wU)-.275 G
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 transport, and for accommodating TFRC as well as TCP-lik)100.8 415 Q(e)
-.11 E(congestion control, we ha)100.8 428 Q .33 -.165(ve c)-.22 H
(oncluded that modi\214cations of SCTP or TCP are not the best).165 E
(answer)100.8 441 Q 2.75(,a)-.44 G(nd that a ne)-2.75 E 2.75(wt)-.275 G
(ransport protocol is needed.)-2.75 E(Thus, we ha)5.5 E .33 -.165(ve a)
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.55 -.275(ew t)-2.75 H(ransport protocol that of).275 E
(fers unreliable deli)-.275 E -.165(ve)-.275 G
(ry; accommodates TFRC as well as).165 E(TCP-lik)100.8 467 Q 2.75(ec)
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(ongestion control; accommodates the use of ECN; and requires minimal)
-2.75 E -.165(ove)100.8 480 S(rhead in pack).165 E
(et size and in the state and CPU processing required at the data recei)
-.11 E -.165(ve)-.275 G -.605(r.).165 G
(This is the line of reasoning that has lead us to the de)100.8 496.6 Q
-.165(ve)-.275 G(lopment of DCCP).165 E 5.5(.B)-1.221 G(oth the)-5.5 E(\
problem statement presented in this document, and the detailed design d\
ecisions made in)100.8 509.6 Q(the design of DCCP)100.8 522.6 Q 2.75(,w)
-1.221 G(ere brought to the IETF for further feedback.)-2.75 E
(This document)5.5 E(addresses only the problem statement; the design d\
ecisions resulting from this problem)100.8 535.6 Q(statement ha)100.8
548.6 Q .33 -.165(ve b)-.22 H(een addressed in later documents [DCCP].)
.165 E F1(4.)72 574.6 Q/F2 14/Times-Bold@0 SF(Selling Congestion Contr)
5.5 E(ol to Reluctant A)-.252 E(pplications)-.35 E F0
(The goal of this w)100.8 591.2 Q(ork is to pro)-.11 E
(vide general congestion control mechanisms that will)-.165 E
(actually be used by man)100.8 604.2 Q 2.75(yo)-.165 G 2.75(ft)-2.75 G
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Q 2.75(yw)-.11 G(ithout end-to-end congestion control.)-2.75 E(Se)5.5 E
-.165(ve)-.275 G(ral of our).165 E(design requirements follo)100.8 630.2
Q 2.75(wf)-.275 G(rom a desire to design and deplo)-2.75 E 2.75(yac)-.11
G(ongestion-controlled)-2.75 E(protocol that is actually attracti)100.8
643.2 Q .33 -.165(ve t)-.275 H 2.75(ot).165 G
(hese "reluctant" applications.)-2.75 E(These design)5.5 E(requirements\
 include the use of Explicit Congestion Noti\214cation \(ECN\) and the \
ECN)100.8 656.2 Q(Nonce, which both pro)100.8 669.2 Q(vide positi)-.165
E .33 -.165(ve b)-.275 H
(ene\214t to the application itself; the choice between).165 E(dif)100.8
682.2 Q(ferent forms of congestion control; and moderate o)-.275 E -.165
(ve)-.165 G(rhead in the size of the pack).165 E(et)-.11 E(header)100.8
695.2 Q(.)-.605 E(There will al)100.8 711.8 Q -.11(wa)-.11 G(ys be a fe)
.11 E 2.75<778d>-.275 G -.275(ow)-2.75 G 2.75(st).275 G
(hat are resistant to the use of end-to-end congestion)-2.75 E
(control, preferring an en)100.8 724.8 Q
(vironment where end-to-end congestion control is used by)-.44 E(Flo)72
769 Q(yd/Handle)-.11 E(y/K)-.165 E 238.51(ohler Section)-.385 F 2.75
(4. [P)2.75 F(age 11])-.165 E 0 Cg EP
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100.8 98 Q(mechanisms are needed to detect and penalize uncontrolled hi\
gh-bandwidth \215o)100.8 111 Q(ws in times)-.275 E
(of high congestion; these router mechanisms are colloquially kno)100.8
124 Q(wn as `penalty box)-.275 E(es'.)-.165 E(Ho)100.8 137 Q(we)-.275 E
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(yment of penalty box)-.11 E(es in)-.165 E
(the Internet, it seems reasonable, and more ef)100.8 150 Q(fecti)-.275
E -.165(ve)-.275 G 2.75(,t).165 G 2.75<6f8c>-2.75 G(rst mak)-2.75 E 2.75
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(Additional Design Considerations)5.5 E F0(This section mentions some a\
dditional design considerations that should be considered in)100.8 205.6
Q(designing a ne)100.8 218.6 Q 2.75(wt)-.275 G(ransport protocol.)-2.75
E 7.15<834d>100.8 235.2 S(obility: Mechanisms for multi-homing and mobi\
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(functionality that cannot necessarily be layered cleanly and ef)111.8
248.2 Q(fecti)-.275 E -.165(ve)-.275 G(ly on top of a).165 E
(transport protocol.)111.8 261.2 Q
(Thus, one outstanding design decision with an)5.5 E 2.75(yn)-.165 G .55
-.275(ew t)-2.75 H(ransport).275 E(protocol concerns whether to incorpo\
rate mechanisms for multi-homing and mobility)111.8 274.2 Q
(into the protocol itself.)111.8 287.2 Q 7.15<8344>100.8 303.8 S
(efense ag)-7.15 E
(ainst DoS attacks and spoo\214ng: A reliable handshak)-.055 E 2.75(ef)
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E(fers protection ag)-.275 E(ainst DoS and spoo\214ng attacks.)-.055 E
(Mechanisms allo)5.5 E(wing)-.275 E 2.75(as)111.8 329.8 S(erv)-2.75 E
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(tate for unackno)-2.75 E(wledged connection attempts or already-)-.275
E(\214nished connections of)111.8 342.8 Q(fer additional protection ag)
-.275 E(ainst DoS attacks.)-.055 E(Thus, in designing)5.5 E 2.75(an)
111.8 355.8 S .55 -.275(ew t)-2.75 H(ransport protocol, e).275 E -.165
(ve)-.275 G 2.75(no).165 G(ne designed to pro)-2.75 E
(vide minimal functionality)-.165 E 2.75(,t)-.715 G(he)-2.75 E
(requirements for pro)111.8 368.8 Q(viding defense ag)-.165 E
(ainst DoS attacks and spoo\214ng need to be)-.055 E(considered.)111.8
381.8 Q 7.15<8349>100.8 398.4 S(nteroperation with R)-7.15 E
(TP: As Section 3.3.3 describes, attention should be paid to an)-.66 E
(y)-.165 E(necessary or desirable changes in R)111.8 411.4 Q
(TP when it is used o)-.66 E -.165(ve)-.165 G 2.75(rt).165 G(he ne)-2.75
E 2.75(wp)-.275 G(rotocol, such as)-2.75 E(reduced R)111.8 424.4 Q
(TP headers.)-.66 E 7.15<8341>100.8 441 S(PI: Some functionality requir\
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(protocol, may require the de\214nition of ne)111.8 454 Q 2.75(wA)-.275
G(PI mechanisms.)-2.75 E(Examples include)5.5 E(allo)111.8 467 Q
(wing applications to decide what information to put in a pack)-.275 E
(et at transmission time,)-.11 E(and pro)111.8 480 Q
(viding applications with some information about pack)-.165 E
(et sequence numbers.)-.11 E 7.15<8349>100.8 496.6 S(nteractions with N)
-7.15 E -1.221(AT)-.385 G 2.75(sa)1.221 G(nd Fire)-2.75 E -.11(wa)-.275
G(lls: N).11 E -1.221(AT)-.385 G 2.75(sa)1.221 G(nd \214re)-2.75 E -.11
(wa)-.275 G(lls don').11 E 2.75(ti)-.198 G(nteract well with UDP)-2.75 E
(,)-1.221 E(with its lack of e)111.8 509.6 Q(xplicit \215o)-.165 E 2.75
(ws)-.275 G(etup and teardo)-2.75 E
(wn and, in practice, the lack of well-kno)-.275 E(wn)-.275 E
(ports for man)111.8 522.6 Q 2.75(yU)-.165 G(DP applications. Some of t\
hese issues are application-speci\214c; others)-2.75 E
(should be addressed by the transport protocol itself.)111.8 535.6 Q
7.15<8343>100.8 552.2 S(onsider general e)-7.15 E
(xperiences with unicast transport: A Requirements for Unicast)-.165 E
-.385(Tr)111.8 565.2 S(ansport/Sessions \(R).385 E(UTS\) BOF w)-.44 E
(as held at the IETF meeting in December)-.11 E 2.75(,1)-.44 G
(998, with)-2.75 E(the goal of understanding the requirements of applic\
ations whose needs were not met by)111.8 578.2 Q(TCP [R)111.8 591.2 Q
(UTS]. Not all of those unmet needs are rele)-.44 E -.275(va)-.275 G
(nt to or appropriate for a unicast,).275 E
(congestion-controlled, unreliable \215o)111.8 604.2 Q 2.75(wo)-.275 G
2.75(fd)-2.75 G(atagrams designed for long-li)-2.75 E -.165(ve)-.275 G
2.75(dt).165 G(ransfers.)-2.75 E(Some are, ho)111.8 617.2 Q(we)-.275 E
-.165(ve)-.275 G .88 -.44(r, a).165 H(nd an).44 E 2.75(yn)-.165 G .55
-.275(ew p)-2.75 H(rotocol should address those needs, and other).275 E
(requirements deri)111.8 630.2 Q -.165(ve)-.275 G 2.75(df).165 G
(rom the community')-2.75 E 2.75(se)-.605 G(xperience. W)-2.915 E 2.75
(eb)-.88 G(elie)-2.75 E .33 -.165(ve t)-.275 H(hat this document).165 E
(addresses the requirements rele)111.8 643.2 Q -.275(va)-.275 G
(nt to our problem area that were brought up at the).275 E -.44(RU)111.8
656.2 S(TS BOF).44 E(.)-.88 E F1(6.)72 682.2 Q F2 -1.036(Tr)5.5 G
(ansport Requir)1.036 E(ements of Request/Response A)-.252 E
(pplications)-.35 E F0(Up until no)100.8 698.8 Q 1.43 -.715(w, t)-.275 H
(his document has discussed the transport and congestion control).715 E
(requirements of applications that generate long-li)100.8 711.8 Q -.165
(ve)-.275 G(d, lar).165 E(ge \215o)-.198 E(ws of unreliable datagrams.)
-.275 E(This section discusses brie\215y the transport needs of another\
 class of applications, those of)100.8 724.8 Q(Flo)72 769 Q(yd/Handle)
-.11 E(y/K)-.165 E 238.51(ohler Section)-.385 F 2.75(6. [P)2.75 F
(age 12])-.165 E 0 Cg EP
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(December 2005)2.75 E(June 2005)116.406 E(request/response transfers wh\
ere the response might be a small number of pack)100.8 85 Q(ets, with)
-.11 E(preferences that include both reliable deli)100.8 98 Q -.165(ve)
-.275 G(ry and a minimum of state maintained at the).165 E 2.75
(ends. The)100.8 111 R(reliable deli)2.75 E -.165(ve)-.275 G
(ry could be accomplished, for e).165 E(xample, by ha)-.165 E
(ving the recei)-.22 E -.165(ve)-.275 G 2.75(rr).165 G(e-)-2.75 E
(query when one or more of the pack)100.8 124 Q
(ets in the response is lost.)-.11 E(This is a class of)5.5 E
(applications whose needs are not well-met by either UDP or by TCP)100.8
137 Q(.)-1.221 E(Although there is a le)100.8 153.6 Q
(gitimate need for a transport protocol for such short-li)-.165 E -.165
(ve)-.275 G 2.75(dr).165 G(eliable)-2.75 E<8d6f>100.8 166.6 Q
(ws of such request/response applications, we belie)-.275 E .33 -.165
(ve t)-.275 H(hat the o).165 E -.165(ve)-.165 G(rlap with the).165 E
(requirements of DCCP is almost non-e)100.8 179.6 Q
(xistent, and that DCCP should not be designed to)-.165 E
(meet the needs of these request/response applications.)100.8 192.6 Q
(Areas of non-compatible)5.5 E(requirements include the follo)100.8
205.6 Q(wing:)-.275 E 7.15<8352>100.8 222.2 S
(eliability: DCCP applications don')-7.15 E 2.75(tn)-.198 G
(eed reliability \(and long-li)-2.75 E -.165(ve)-.275 G 2.75(da).165 G
(pplications that do)-2.75 E
(require reliability are well-suited to TCP or SCTP\).)111.8 235.2 Q
(In contrast, these short-li)5.5 E -.165(ve)-.275 G(d).165 E(request/re\
sponse applications do require reliability \(possibly client-dri)111.8
248.2 Q -.165(ve)-.275 G 2.75(nr).165 G(eliability in)-2.75 E
(the form of requesting missing se)111.8 261.2 Q
(gments of a response\).)-.165 E 7.15<8343>100.8 277.8 S
(onnection setup and teardo)-7.15 E(wn: Because DCCP is aimed at \215o)
-.275 E(ws whose duration is)-.275 E(often unkno)111.8 290.8 Q
(wn in adv)-.275 E(ance, it addresses interactions with N)-.275 E -1.221
(AT)-.385 G 2.75(sa)1.221 G(nd \214re)-2.75 E -.11(wa)-.275 G(lls by ha)
.11 E(ving)-.22 E -.165(ex)111.8 303.8 S(plicit handshak).165 E
(es for setup and teardo)-.11 E 2.75(wn. In)-.275 F
(contrast, the short-li)2.75 E -.165(ve)-.275 G 2.75(dr).165 G
(equest/response)-2.75 E(applications kno)111.8 316.8 Q 2.75(wt)-.275 G
(he transfer length in adv)-2.75 E(ance, b)-.275 E
(ut cannot tolerate the additional delay)-.22 E(of a handshak)111.8
329.8 Q 2.75(ef)-.11 G(or \215o)-2.75 E 2.75(ws)-.275 G 2.75
(et-up. Thus,)-2.75 F(mechanisms for interacting with N)2.75 E -1.221
(AT)-.385 G 2.75(sa)1.221 G(nd)-2.75 E(\214re)111.8 342.8 Q -.11(wa)
-.275 G(lls are lik).11 E(ely to be completely dif)-.11 E
(ferent for the tw)-.275 E 2.75(os)-.11 G(ets of applications.)-2.75 E
7.15<8343>100.8 359.4 S(ongestion-control mechanisms: The styles of con\
gestion control mechanisms and)-7.15 E(ne)111.8 372.4 Q
(gotiations of congestion control features are hea)-.165 E
(vily dependent on the \215o)-.22 E 2.75(wd)-.275 G(uration.)-2.75 E(In\
 addition, the preference of the request/response applications for a st\
ateless serv)111.8 385.4 Q(er)-.165 E
(strongly impacts the congestion control choices.)111.8 398.4 Q
(Thus, DCCP and the short-li)5.5 E -.165(ve)-.275 G(d).165 E
(request/response applications ha)111.8 411.4 Q .33 -.165(ve r)-.22 H
(ather dif).165 E(ferent requirements both for congestion)-.275 E
(control mechanisms and for ne)111.8 424.4 Q(gotiation procedures.)-.165
E/F1 11/Times-Bold@0 SF(7.)72 450.4 Q/F2 14/Times-Bold@0 SF
(Summary of Recommendations)5.5 E F0(Our problem statement has discusse\
d the need for implementing congestion control for)100.8 467 Q
(unreliable \215o)100.8 480 Q 2.75(ws. Additional)-.275 F
(problems concern the need for lo)2.75 E 2.75(wo)-.275 G -.165(ve)-2.915
G(rhead, the problems of).165 E(\214re)100.8 493 Q -.11(wa)-.275 G
(ll tra).11 E -.165(ve)-.22 G
(rsal, and the need for reliable parameter ne).165 E 2.75
(gotiation. Our)-.165 F(consideration of the)2.75 E
(problem statement has resulted in the follo)100.8 506 Q
(wing general recommendations:)-.275 E 7.15<8341>100.8 522.6 S
(unicast transport protocol for unreliable datagrams should be de)-4.4 E
-.165(ve)-.275 G(loped, including).165 E(mandatory)111.8 535.6 Q 2.75
(,b)-.715 G(uilt-in congestion control, e)-2.97 E
(xplicit connection setup and teardo)-.165 E(wn, reliable)-.275 E
(feature ne)111.8 548.6 Q(gotiation, and reliable congestion feedback.)
-.165 E 7.15<8354>100.8 565.2 S(he protocol must pro)-7.15 E
(vide a set of congestion control mechanisms from which the)-.165 E(app\
lication may choose. These mechanisms should include, at minimum, TCP-l\
ik)111.8 578.2 Q(e)-.11 E(congestion control and a more slo)111.8 591.2
Q(wly-responding congestion control such as TFRC.)-.275 E 7.15<8349>
100.8 607.8 S(mportant features of the connection, such as the congesti\
on control mechanism in use,)-7.15 E(should be reliably ne)111.8 620.8 Q
(gotiated by both endpoints.)-.165 E 7.15<8353>100.8 637.4 S
(upport for ECN should be included.)-7.15 E
(\(Applications could still mak)5.5 E 2.75(et)-.11 G(he decision not to)
-2.75 E(use ECN for a particular session.\))111.8 650.4 Q 7.15<8354>
100.8 667 S(he o)-7.15 E -.165(ve)-.165 G(rhead must be lo).165 E 1.43
-.715(w, i)-.275 H 2.75(nt).715 G(erms of both pack)-2.75 E
(et size and protocol comple)-.11 E(xity)-.165 E(.)-.715 E 7.15<8353>
100.8 683.6 S(ome DoS protection for serv)-7.15 E
(ers must be included. In particular)-.165 E 2.75(,s)-.44 G(erv)-2.75 E
(ers can mak)-.165 E(e)-.11 E(themselv)111.8 696.6 Q
(es resistant to spoofed connection attacks \("SYN \215oods"\).)-.165 E
(Flo)72 769 Q(yd/Handle)-.11 E(y/K)-.165 E 238.51(ohler Section)-.385 F
2.75(7. [P)2.75 F(age 13])-.165 E 0 Cg EP
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(December 2005)2.75 E(June 2005)116.406 E 7.15<8343>100.8 85 S
(onnection setup and teardo)-7.15 E(wn must use e)-.275 E
(xplicit handshak)-.165 E(es, f)-.11 E(acilitating transmission)-.11 E
(through stateful \214re)111.8 98 Q -.11(wa)-.275 G(lls.).11 E
(In 2002, there w)100.8 114.6 Q
(as judged to be a consensus about the need for a ne)-.11 E 2.75(wu)
-.275 G(nicast transport)-2.75 E
(protocol for unreliable datagrams, and the ne)100.8 127.6 Q(xt step w)
-.165 E(as then the consideration of more)-.11 E
(detailed architectural issues.)100.8 140.6 Q/F1 11/Times-Bold@0 SF(8.)
72 166.6 Q/F2 14/Times-Bold@0 SF(Ackno)5.5 E(wledgements)-.14 E F0 1.76
-.88(We w)100.8 183.2 T(ould lik).77 E 2.75(et)-.11 G 2.75(ot)-2.75 G
(hank Spencer Da)-2.75 E(wkins, Jiten Goel, Jef)-.165 E 2.75(fH)-.275 G
(ammond, Lars-Erik Jonsson,)-2.75 E(John Loughne)100.8 196.2 Q 1.43
-.715(y, M)-.165 H(ichael Mealling, and Rik W).715 E
(ade for feedback on earlier v)-.88 E(ersions of this)-.165 E 2.75
(document. W)100.8 209.2 R 2.75(ew)-.88 G(ould also lik)-2.86 E 2.75(et)
-.11 G 2.75(ot)-2.75 G(hank members of the T)-2.75 E(ransport Area W)
-.385 E(orking Group)-.88 E(and of the DCCP W)100.8 222.2 Q
(orking Group for discussions of these issues.)-.88 E F1(9.)72 248.2 Q
F2(Inf)5.5 E(ormati)-.35 E .28 -.14(ve R)-.14 H(efer).14 E(ences)-.252 E
F0([Bala99] H. Balakrishnan, H. Rahul, and S. Seshan, An Inte)100.8
264.8 Q(grated Congestion)-.165 E
(Management Architecture for Internet Hosts, SIGCOMM, Sept. 1999.)129.6
277.8 Q([CCID 2 PR)100.8 294.4 Q(OFILE] S. Flo)-.44 E(yd and E. K)-.11 E
(ohler)-.385 E 2.75(.P)-.605 G
(ro\214le for DCCP Congestion Control ID 2:)-2.75 E(TCP-lik)129.6 307.4
Q 2.75(eC)-.11 G(ongestion Control, draft-ietf-dccp-ccid2-08.txt, w)
-2.75 E(ork in progress,)-.11 E(No)129.6 320.4 Q -.165(ve)-.165 G
(mber 2004.).165 E([CCID 3 PR)100.8 337 Q(OFILE] S. Flo)-.44 E(yd, E. K)
-.11 E(ohler)-.385 E 2.75(,a)-.44 G(nd J. P)-2.75 E(adh)-.165 E 2.75
(ye. Pro\214le)-.055 F(for DCCP Congestion)2.75 E
(Control ID 3: TFRC Congestion Control.)129.6 350 Q
(draft-ietf-dccp-ccid3-08.txt, w)5.5 E(ork in)-.11 E(progress, No)129.6
363 Q -.165(ve)-.165 G(mber 2004.).165 E([DCCP] E. K)100.8 379.6 Q
(ohler)-.385 E 2.75(,M)-.44 G 2.75(.H)-2.75 G(andle)-2.75 E 1.43 -.715
(y, a)-.165 H(nd S. Flo).715 E 2.75(yd. Datagram)-.11 F
(Congestion Control Protocol,)2.75 E(draft-ietf-dccp-spec-09.txt, w)
129.6 392.6 Q(ork in progress, No)-.11 E -.165(ve)-.165 G(mber 2004.)
.165 E([MEASWEB] Ramon Caceres and Sally Flo)100.8 409.2 Q
(yd, Measurement Studies of End-to-End)-.11 E
(Congestion Control in the Internet, W)129.6 422.2 Q(eb P)-.88 E
(age, 2001.)-.165 E([FF99] S. Flo)100.8 438.8 Q(yd and K. F)-.11 E
(all, Promoting the Use of End-to-End Congestion Control in the)-.165 E
(Internet, IEEE/A)129.6 451.8 Q(CM T)-.44 E(ransactions on Netw)-.385 E
(orking, August 1999.)-.11 E([MC01] S. McCreary and K.C. Claf)100.8
468.4 Q(fy)-.275 E 2.75(,T)-.715 G(rends in W)-3.135 E(ide Area IP T)
-.44 E(raf)-.385 E(\214c P)-.275 E(atterns: A V)-.165 E(ie)-.66 E(w)
-.275 E(from Ames Internet Exchange, ITC Specialist Seminar)129.6 481.4
Q 2.75(,2)-.44 G 2.75(001. URL)-2.75 F -.814(``)129.6 494.4 S
(http://www).814 E(.caida.or)-.715 E(g/outreach/papers/2000/AIX0005/')
-.198 E('.)-.814 E([RFC 2026] S. Bradner)100.8 511 Q 2.75(.T)-.605 G
(he Internet Standards Process -- Re)-2.75 E(vision 3. RFC 2026.)-.275 E
([RFC 2481] K. Ramakrishnan, S. Flo)100.8 527.6 Q
(yd. A Proposal to add Explicit Congestion)-.11 E
(Noti\214cation \(ECN\) to IP)129.6 540.6 Q 2.75(.R)-1.221 G(FC 2481.)
-2.75 E([RFC 1191] J. C. Mogul, S. E. Deering. P)100.8 557.2 Q
(ath MTU Disco)-.165 E -.165(ve)-.165 G(ry).165 E 2.75(.R)-.715 G
(FC 1191.)-2.75 E([RFC 2309] B. Braden et al., Recommendations on Queue\
 Management and Congestion)100.8 573.8 Q -1.342 -.814(Av o)129.6 586.8 T
(idance in the Internet.).814 E(RFC 2309, April 1998.)5.5 E
([RFC 2525] V)100.8 603.4 Q 2.75(.P)-1.419 G(axson et al., Kno)-2.915 E
(wn TCP Implementation Problems, RFC 2525, March)-.275 E(1999.)129.6
616.4 Q([RFC 2914] S. Flo)100.8 633 Q
(yd. Congestion Control Principles.)-.11 E(RFC 2914, Sept. 2000.)5.5 E
([RFC 2960] R. Ste)100.8 649.6 Q -.11(wa)-.275 G
(rt, Q. Xie, K. Morneault, C. Sharp, H. Schw).11 E(arzbauer)-.11 E 2.75
(,T)-.44 G 2.75(.T)-3.564 G(aylor)-3.63 E 2.75(,I)-.44 G(.)-2.75 E
(Rytina, M. Kalla, L. Zhang, V)129.6 662.6 Q 2.75(.P)-1.419 G
(axson. Stream Control T)-2.915 E(ransmission Protocol. RFC)-.385 E
(2960.)129.6 675.6 Q
([RFC 3124] H. Balakrishnan, S. Seshan. The Congestion Manager)100.8
692.2 Q 2.75(.R)-.605 G(FC 3124.)-2.75 E([RFC 3448] M. Handle)100.8
708.8 Q 1.43 -.715(y, S)-.165 H 2.75(.F).715 G(lo)-2.75 E(yd, J. P)-.11
E(adh)-.165 E(ye, and J. W)-.055 E(idmer)-.44 E 5.5(.T)-.605 G
(CP Friendly Rate Control)-5.5 E(\(TFRC\): Protocol Speci\214cation.)
129.6 721.8 Q(RFC 3448, January 2003.)5.5 E(Flo)72 769 Q(yd/Handle)-.11
E(y/K)-.165 E 238.51(ohler Section)-.385 F 2.75(9. [P)2.75 F(age 14])
-.165 E 0 Cg EP
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(December 2005)2.75 E(June 2005)116.406 E([RFC 3540] D. W)100.8 85 Q
(etherall, D. Ely)-.88 E 2.75(,a)-.715 G(nd N. Spring. Rob)-2.75 E
(ust ECN Signaling with Nonces, June)-.22 E(2003.)129.6 98 Q([R)100.8
114.6 Q(UTS] Requirements for Unicast T)-.44 E(ransport/Sessions \(R)
-.385 E(UTS\) BOF)-.44 E 2.75(,D)-.88 G 2.75(ec. 7,)-2.75 F 2.75
(1998. URL)2.75 F("http://www)129.6 127.6 Q(.ietf.or)-.715 E
(g/proceedings/98dec/43rd-ietf-98dec-142.html".)-.198 E([TBIT] J. P)
100.8 144.2 Q(adh)-.165 E(ye and S. Flo)-.055 E
(yd, Identifying the TCP Beha)-.11 E(vior of W)-.22 E(eb Serv)-.88 E
(ers, SIGCOMM)-.165 E(2001.)129.6 157.2 Q/F1 11/Times-Bold@0 SF(10.)72
183.2 Q/F2 14/Times-Bold@0 SF -.7(Au)5.5 G(thors' Addr).7 E(esses)-.252
E F0(Sally Flo)100.8 199.8 Q(yd <\215o)-.11 E(yd@icir)-.11 E(.or)-.605 E
(g>)-.198 E(Mark Handle)100.8 212.8 Q 2.75(y<)-.165 G(mjh@icir)-2.75 E
(.or)-.605 E(g>)-.198 E(Eddie K)100.8 225.8 Q(ohler <k)-.385 E
(ohler@icir)-.11 E(.or)-.605 E(g>)-.198 E
(ICSI Center for Internet Research \(ICIR\),)100.8 251.8 Q
(International Computer Science Institute,)100.8 264.8 Q
(1947 Center Street, Suite 600)100.8 277.8 Q(Berk)100.8 290.8 Q(ele)-.11
E 1.43 -.715(y, C)-.165 H 2.75(A9).715 G(4704.)-2.75 E F1(11.)72 316.8 Q
F2(Full Copyright Statement)5.5 E F0(Cop)100.8 333.4 Q
(yright \(C\) The Internet Society 2005.)-.11 E
(This document is subject to the rights, licenses)5.5 E
(and restrictions contained in BCP 78, and e)100.8 346.4 Q
(xcept as set forth therein, the authors retain all)-.165 E
(their rights.)100.8 359.4 Q
(This document and the information contained herein are pro)100.8 376 Q
(vided on an "AS IS" basis and)-.165 E(THE CONTRIB)100.8 389 Q(UT)-.11 E
(OR, THE ORGANIZA)-.198 E(TION HE/SHE REPRESENTS OR IS)-1.221 E
(SPONSORED BY \(IF ANY\), THE INTERNET SOCIETY AND THE INTERNET)100.8
402 Q(ENGINEERING T)100.8 415 Q(ASK FORCE DISCLAIM ALL W)-1.023 E
(ARRANTIES, EXPRESS OR)-1.32 E(IMPLIED, INCLUDING B)100.8 428 Q(UT NO)
-.11 E 2.75(TL)-.44 G(IMITED T)-2.75 E 2.75(OA)-.198 G(NY W)-2.75 E
(ARRANTY THA)-1.32 E 2.75(TT)-1.221 G(HE USE)-2.75 E(OF THE INFORMA)
100.8 441 Q(TION HEREIN WILL NO)-1.221 E 2.75(TI)-.44 G
(NFRINGE ANY RIGHTS OR ANY)-2.75 E(IMPLIED W)100.8 454 Q
(ARRANTIES OF MERCHANT)-1.32 E(ABILITY OR FITNESS FOR A)-1.023 E -1.012
(PA)100.8 467 S -.66(RT)1.012 G(ICULAR PURPOSE.).66 E F1(12.)72 493 Q F2
(Intellectual Pr)5.5 E(operty)-.252 E F0(The IETF tak)100.8 509.6 Q
(es no position re)-.11 E -.055(ga)-.165 G(rding the v).055 E
(alidity or scope of an)-.275 E 2.75(yI)-.165 G(ntellectual Property)
-2.75 E(Rights or other rights that might be claimed to pertain to the \
implementation or use of the)100.8 522.6 Q
(technology described in this document or the e)100.8 535.6 Q
(xtent to which an)-.165 E 2.75(yl)-.165 G(icense under such rights)
-2.75 E(might or might not be a)100.8 548.6 Q -.275(va)-.22 G
(ilable; nor does it represent that it has made an).275 E 2.75(yi)-.165
G(ndependent)-2.75 E(ef)100.8 561.6 Q(fort to identify an)-.275 E 2.75
(ys)-.165 G(uch rights.)-2.75 E
(Information on the procedures with respect to rights in)5.5 E
(RFC documents can be found in BCP 78 and BCP 79.)100.8 574.6 Q
(Copies of IPR disclosures made to the IETF Secretariat and an)100.8
591.2 Q 2.75(ya)-.165 G(ssurances of licenses to)-2.75 E(be made a)100.8
604.2 Q -.275(va)-.22 G(ilable, or the result of an attempt made to obt\
ain a general license or).275 E(permission for the use of such propriet\
ary rights by implementers or users of this)100.8 617.2 Q(speci\214cati\
on can be obtained from the IETF on-line IPR repository at)100.8 630.2 Q
(http://www)100.8 643.2 Q(.ietf.or)-.715 E(g/ipr)-.198 E(.)-.605 E
(The IETF in)100.8 659.8 Q(vites an)-.44 E 2.75(yi)-.165 G
(nterested party to bring to its attention an)-2.75 E 2.75(yc)-.165 G
(op)-2.75 E(yrights, patents or)-.11 E
(patent applications, or other proprietary rights that may co)100.8
672.8 Q -.165(ve)-.165 G 2.75(rt).165 G(echnology that may be)-2.75 E
(required to implement this standard.)100.8 685.8 Q
(Please address the information to the IETF at ietf-)5.5 E(ipr@ietf.or)
100.8 698.8 Q(g.)-.198 E(Flo)72 769 Q(yd/Handle)-.11 E(y/K)-.165 E
233.01(ohler Section)-.385 F 2.75(12. [P)2.75 F(age 15])-.165 E 0 Cg EP
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(y/K)-.165 E 233.01(ohler Section)-.385 F 2.75(12. [P)2.75 F(age 16])
-.165 E 0 Cg EP
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